1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a two-gate structure that permits enhanced off-current suppression in a top gate thin-film transistor (TFT).
2. Description of the Related Art
FIGS. 1A and 1B are, respectively, partial cross-sectional views of bottom gate and top gate TFT devices made using an amorphous silicon (Si) active layer (prior art). For ease of fabrication using well-established process flows, the channel is formed from a thin-film material that is interposed between the source and drain, while overlapping the regions (contact regions) of the source and drain. Such a structure is optimal for an amorphous Si (a-Si) active layer because the gate/contact overlap ensures low contact resistance without causing high off-current.
To economically fabricate higher quality consumer devices such a liquid crystal display (LCD) televisions, so-called mid-mobility TFTs may be fabricated over glass panel substrates using mid-mobility materials (e.g. microcrystalline silicon (μc-Si)) as the active layer, in place of more conventional materials such as a-Si. In addition to the higher effective mobility due to better quality active layer, these TFT's are required to have a similar or lower level of off-current. Except for this active layer deposition step, it is desirable that the devices are fabricated using conventional TFT process technology. The use of these mid-mobility devices could provide a technical path to the integration of a variety of circuits and address the so-called system-on-panel concept.
However, the above-described channel contact structure is not necessarily optimal for use with mid-mobility active layers made from μc-Si. When μc-Si replaces a-Si as active layer in the conventional structure, operation in the off-state subjects the μc-Si active layer to very large field. The smaller energy gap, higher mobility, and large defect density in the active film result in off-current levels that are much higher than if a-Si is used. Alternately stated, the simplicity of conventional structures and processes puts constraints on the off-current parameter, because of the large overlap between gate and contact regions.
Thus, if a mid-mobility material such as μc-Si is to be used in place of a-Si, an alternative structure is needed to address the issue of high off-state current due to field-enhanced carrier generation. Without the capability of suppressing the off-current, an overall increase in the current ON/OFF ratio cannot be realized.
It would be advantageous if a TFT structure could be devised that minimized the electric field influencing the active layer at the contact region. It would be advantageous if the improved TFT had a lower off-current than a conventional top-gate TFT.